Inkjet printing is a preferred technology for printing color images. Both continuous inkjet and drop on demand inkjet methods are commonly practiced. In commercial inkjet printers of both types, drops of ink expelled from a printhead traverse a short distance in air to a receiver on which they land, thereby producing a visible image on the receiver. Continuous inkjet printing methods rely on directional control of a stream of continuously produced droplets, while drop on demand methods rely on thermal drop expulsion (as embodied by products from Hewlett Packard Co. and Canon Corp., for example) and on piezo drop expulsion (as embodied by products from Epson Corp., for example). Such inkjet printers suffer from certain drawbacks, for example the difficulty of positioning drops accurately and inexpensively on the receiver. Also, there is generally a need to precisely move or scan the printhead with respect to the receiver on which the droplets land. Mechanical mechanisms to accomplish this motion are costly, require substantial power to operate, and take up space; considerations particularly important for the low cost portable printers. The principally know means of providing continuous tone color reproduction, namely the deposition of multiple drops onto a single image pixel, suffers from an uncertainty in the exact location of the printed pixels because the receiver is typically moving during printing and multiple drops cannot be released simultaneously.
Inkjet printers as currently practiced also suffer from a difficulty of inexpensively achieving continuous tone (grayscale) color reproduction. Such grayscale color reproduction is well known in the art of color printing to be advantageous in producing high quality images. Although some printers control the volume of drops, only drops of a particular color are deposited on the receiver at any one time, and the resulting tone scale is not ideal, because in the case of deposition of two or more ink colors, the first color has dried or been absorbed by the receiver appreciably before drops of the second color are deposited. Also, such methods of continuous tone color reproduction suffer image artifacts because the less dense image pixels, corresponding to smaller volumes of ink, do not occupy the same area on the receiver as the higher density image pixels, corresponding to larger volumes of ink. Failure to print pixels of equal area regardless of image density is known to produce visual artifacts in printed images.
Some solutions to these problems have been proposed in commonly assigned U.S. patent application Ser. No. 08/882,620, in which ink is deposited on a receiver without the need for the drops to traverse a distance in air to the receiver. According to the contact printhead array disclosed, a substrate is provided with a multiplicity of ink channels and ink in each ink channel is pumped by a corresponding multiplicity of pumps directly to a receiver in contact with the openings of the ink channels at the substrate top surface. Such a contact printhead array comprises a two dimensional array of such ink channels and pumps in order to print all image pixels without the necessity of movement of the receiver with respect to the printhead. Also disclosed are chambers for mixing of inks of different colors prior to deposition of the mixed inks on a receiver, aimed at improving color image quality.
Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069, 5,593,838, and 5,603,351. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and them pumped into a bottom array of reaction cells. The array may be decoupled from the assembly and removed for incubation or analysis. When used as a printing device, the chemical reagent solutions are replaced by dispersions of cyan, magenta, and yellow pigment, and the array of reaction cells may be considered a viewable display of picture elements, or pixels, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of the paper fibers pulls the dye from the cells and holds it in the paper, thus producing a paper print, or photograph, of the original scene. One problem with this kind of printer is the rendering of an accurate tone scale. The problem comes about because the capillary force of the paper fibers remove all the pigment solution from the cell, draining it empty. If, for example, a yellow pixel is being printed, the density of the image will be fully yellow. However, in some scenes, a light, or pale yellow is the original scene color. One way to solve this problem might be to stock and pump a number of yellow pigments ranging from very light to dark yellow. Another way to solve the tone scale problem is to print a very small dot of dark yellow and leave white paper surrounding the dot. The human eye will integrate the white and the small dot of dark yellow leading to an impression of light yellow, provided the dot is small enough. This is the principle upon which the art of color halftone lithographic printing rests. It is sometimes referred to as area modulation of tone scale. However, in order to provide a full tone scale of colors, a high resolution printer is required, with many more dots per inch than would be required if the colors could be printed at different densities. Another solution to the tone scale problem has been provided in the area of ink jet printers, as described in U.S. Pat. No. 5,606,351, by Gilbert A. Hawkins, hereby incorporated by reference. In an ink jet printer, the drop size is determined primarily by the surface tension of the ink and the size of the orifice from which the drop is ejected. The ink jet printer thus has a similar problem with rendition of tone scale. The Hawkins patent overcomes the problem by premixing the colored ink with a colorless ink in the correct proportions to produce a drop of ink of the correct intensity to render tone scale. However, ink jet printers require a relatively high level of power to function, and they tend to be slow since only a few pixels are printed at a time (serial printing), in comparison to the microfluidic printer in which all the pixels are printed simultaneously (parallel printing). Also, displays for viewing the image before printing, i.e. LCDs, CRTs, require cost and power that make incorporating them in a portable device impractical.
Such contact printhead arrays are however difficult to fabricate inexpensively due to the size and complexity of the ink channels, pumps, and mixing chambers, particularly for the printing of high quality images with closely spaced pixels, for examples pixels spaced more closely than about 100 microns. As is well known in the art, there is a need for more closely spaced pixels. High quality images are typically printed in the range of from 300 to 2400 dots per inch, the commonly used measure of the density of image pixels, corresponding to pixel spacings of from 80 to 10 microns. Also, the degree of mixing of fluids in mixing chambers is subject to variations due to the time of residence of fluids in the chambers, the order and timing of the combination of the fluids, as is well know in the art of microfluidic mixing, and is disadvantageous for the consistent reproduction of color hue and saturation.